As in, why *isn't* there a minimum ilvl requirement for leveling dungeons? (Will talk about ARR content later), tldr at bottom.
I'm not asking for i270 for Sirensong.
I'm asking to not have i120 tank in Doma. Not to have an i200 in Sirensong. Not to have i110 in The Vault.
With gear so poor, you are an active detriment to your party. This isn't a single player game.
With the constant gear treadmill in the Main Story Quest, there's no legitimate reason to NOT be at an acceptable ilvl. They even force you to go out of your way to increase ilvl extremely early during the MSQ, and in your later jobs, you have the resources to gear through alternative means.
Gear so poor either makes the dungeon impossible to complete (in the time allotted, in most cases) in the case of a tank or healer, or as a dps, it makes you worthless. A healer should not be auto-attacking for more than your damage, your "effort" doesn't matter, your effort should have been placed into acquiring proper gear. The mobs don't care if you're "trying your best", they aren't magically going to take more damage. You should have tried your best at the Market Board or at the NPC vendor that sells level appropriate gear.
As for ARR content, it has a different issue. Due to lack of early-game funds and low level accessories, you can't just slap ilvl requirements onto ARR dungeons. Instead I ask for different fix. DoW/All Class gear. Tanks are not educated and the game isn't forcing them to use proper gear. If you go into Brayflox or Stone Vigil with DoW gear, you are going to get wrecked.
When I went into Stone Vigil in Level 40 NQ tank vendortrash, I had 393 defense, including the Brand-New Ring. Now if I went in with DoW gear of the same level, I'd have 192. Less than half. Unacceptable, has to be fixed. This problem has not gone away with time, there are still new tanks that think this is okay gear. If you don't force them to use the right gear, they wont.
TLDR
I'm not asking for people to have gear equal to the ilvl the dungeon drops.
I'm asking for ilvl equal to what the vendor sells at the level of the dungeon's unlock (or equal to the prior dungeon's gear drops)
If you don't have the gil to afford vendortrash, go do fates, sidequests, or leves to get the paltry sum required. You aren't the judge to declare your time more valuable than that of 3 other people.
EDIT - And it should go without saying that we should also start requiring jobs in the Duty Finder for all content higher than 30.